1. Field of the Invention
The present invention relates to sinkers and, more particularly, to a sinker with a reduced width.
2. Description of the Related Art
A sinker is a heavily-doped region, such as an n+ region, that extends from the top surface of a semiconductor body, such as an epitaxial layer, down a substantial distance into the semiconductor body. A sinker can be used, for example, to provide a low resistance current path, such as the collector sinker of a bipolar transistor. Sinkers can also be used to provide lateral isolation for devices that are formed in the semiconductor body.
FIG. 1 shows a cross-sectional view that illustrates a prior-art semiconductor structure 100. As shown in FIG. 1, semiconductor structure 100 includes an epitaxial layer 110, and a sinker 112 that extends down into epitaxial layer 110. Sinker 112 has a depth X of approximately 5-10 μm, an n conductivity type, a peak dopant concentration greater than 1×1019 atoms/cm3, and a sheet resistance less than approximately 5-10 ohms/square.
Conventionally, sinkers, such as sinker 112, are fabricated by first forming a patterned hard mask on the semiconductor body, such as epitaxial layer 110. The patterned hard mask has an opening that extends through the hard mask. A dopant, such as an n-type dopant, is then implanted through the opening into the semiconductor body. The implanted dopant has a width Y that is defined by the width of the opening in the hard mask. For example, the width Y can be approximately 1 μm. Following this, the implanted dopant is driven in to form the sinker.
One problem with the conventional formation of sinkers is that, following the drive in, the resulting sinkers are quite large and consume a significant amount of silicon real estate. Although the width Y of the dopant may be approximately 1 μm following the implant, the sinker expands in size to have a width Z that is much larger than the width Y as a result of diffusion from the drive in. For example, the width Z can be greater than 10 μm.
Sinkers with large widths limit the number of laterally-adjacent devices that can be formed in the semiconductor body. As a result, there is a need to reduce the widths of the sinkers.